A "world war" seems way too extreme for 1 AD. It's Antiquity, so a small, localized conflict would be the norm. The Guardia and Porre domains would probably be very small, around the Denadoro region perhaps. Generally, for the Zenanians (ehe, I think I coined a term), Medina would be like India for the Romans (= known, but rarely in contact with), and Choras would basically be an unknown continent. Everything that isn't Guardia/Porre would be unrelated to their war.
Maybe we could make the 1 AD Choras a sort of Aborigine Australia (even if it's snowy!), since the Chrono series always had a subtle but rather fitting Aborigine religion feeling (see
the Dreamtime and beliefs about dreams being able to shape the planet), especially with Schala's speech during the CC ending.
Well, I don't really know
how this Choras would fit with the plot actually, even though even in CT, Choras had not much to do with the main plot anyway. Just throwing ideas.
As for the other overworlds, if one is the "alternate world", and one is 1 AD... I guess there's only one left, the Future era. But yeah, the problem is the tileset. Computer screens and hot hi-tech devices are cruelly missing. This is quite an issue in practise.